Biochemical and functional studies on EMD 49,980: a potent, selectively presynaptic D-2 dopamine agonist with actions...

EMD 49,980 proved to be a potent and selectively presynaptic D-2 dopamine receptor agonist in biochemical studies with rats. Thus, the gamma-butyrolactone-induced accumulation of dihydroxyphenylalanine, used as a presynaptic model, was antagonized with ED50 values of 0.29 and 0.09 mumol/kg in striatum and t. olfactorium, respectively, with high maximal effects. In contrast, striatal acetylcholine concentrations, reflecting actions at normosensitive postsynaptic D-2 receptors, were only moderately increased by about 30% over a dose range of 2.3-68 mumol/kg. In rats with unilateral nigrostriatal lesions, EMD 49,980 induced long-lasting contralateral turning, indicative of agonistic actions at denervated postsynaptic D-2 receptors. In addition, EMD 49,980 potently inhibited serotonin (5-HT) uptake in vitro and in vivo. Binding studies confirmed D-2 activity in the nM range but, similarly potent effects were observed at 5-HT1A binding sites. Measurement of 5-hydroxytryptophan (5-HTP) accumulation in the n. raphe revealed that, in vivo, the net effect of EMD 49,980 on 5-HT systems is an agonistic one. Control experiments indicate that inhibition of 5-HTP accumulation by EMD 49,980 is induced mainly via direct activation of 5-HT1A receptors, although some contribution due to 5-HT uptake inhibition is likely. Furthermore, results with various reference compounds make it unlikely that there are indirect effects, also via alpha 2-receptors in the models used and support the view that D-2 agonistic, 5-HT uptake inhibiting and 5-HT1A agonistic actions are independent properties of EMD 49,980.